High quality hydrogen terminated Si (111) surfaces provide us
with a new
material system to study a two-dimensional electron system with
multi-valley
interactions. In our field effect structure where a H-Si(111)
substrate is
bonded to a SOI substrate, two-dimensional electrons are confined
at the
hydrogen-terminated Si(111) surface with a vacuum barrier. In our
previous
work, a high-mobility ($\mu $=110,00cm2/Vs) sample shows sixfold
degeneracy,[1] while on a sample with $\mu $=24,000cm2/Vs, the
sixfold
degeneracy is broken.[2] In order to find out the relationship
between the
electron mobility and the valley splitting, we have investigated
a number of
devices with mobility ($\mu $=10,000$\sim $25,000 cm2/Vs), and
observed that
most of them show the sixfold degeneracy, while a few of them
show large
asymmetry. Possible explanations will be presented. We will also
compare the
temperature dependence of the electron mobility between the
high-mobility
sample and a moderate-mobility sample, and discuss the possible
different
limiting factors behind them. [1] R. N. McFarland et al.,
Phys.Rev.B 161310R
(2009). [2] K. Eng, et al., Phys. Rev. Lett. 99, 016801 (2007). [Preview Abstract]

The magnetoresistance $\rho_{xx}$ and $\rho_{xy}$ as well as
attenuation and velocity change of surface acoustic waves were
measured in a p-Si/SiGe sample with $p$=2$\times$10$^{11}$\,cm$^
{-2}$. The research was performed in the temperature range of
0.3-2 K and in the magnetic fields of up to 18 T tilted with
respect to the two-dimensional (2D) channel plane. The
dependence of the g-factor g*($\Theta$)/g*($0^{\circ}$) on the tilt
angle was determined. The measurements of $\rho_{xx}$ and
$\rho_{xy}$ in the tilted magnetic field showed that the
anomaly in $\rho_{xx}$ observed at filling factor $\nu$ =3/2 is
insignificant in the conductivity $\sigma_{xx}$. The anomaly in
$\sigma_{xx}$ at $\nu$ =2 might be explained by overlapping of
the levels with different spins 0$\uparrow$ and 1$\downarrow$
when the tilt angle of the applied magnetic field is changed.
The overlapping occurs at $\Theta$ of about $60^{\circ}$ and causes
a ferromagnetic-paramagnetic transition. [Preview Abstract]

The clean $\nu = 1$ quantum Hall bilayer is an excitonic
superfluid. Experimentally, due to disorder,
the counterflow conductivity $\sigma_{CF}$
remains finite even at the lowest $T$ and the zero-bias peak
has
finite width.
We mimic the nonperturbative effects of disorder [1] by a
periodic potential[2] which couples,
in a spin-only model, to the topological density ${\mathbf n}
\cdot{\partial_x{\mathbf
n}}\times{\partial_y{\mathbf n}}$(=charge density). We find a
set
of ground state phase transitions as the potential strength
increases, with increasing local charge
density. The transitions are weakly first order, with a new,
quadratically dispersing, charge-carrying
mode which represent incipient meron-antimeron pairs forming in
regions of large
potential gradient. These modes
can become nearly gapless at and near the transition,
which we argue leads to a strong
suppression of the interlayer tunneling h.
We demonstrate that
near the transitions vortex-antivortex pairs become easy
to create, leading to a strong suppression of $T_{KT}$ . We
discuss an effective theory
that incorporates both
the Goldstone
mode and the new, quadratically dispersion mode.
\\[4pt]
[1] H. A. Fertig and G. Murthy, prl {\bf 95}, 156802 (2005).\\[0pt]
[2] G. Murthy and S. Sachdev, prl {\bf 101}, 226801 (2008). [Preview Abstract]

We develop a theory of transverse thermoelectric (Peltier)
conductivity, $\alpha_{xy}$, in finite magnetic field -- this
particular conductivity is often the most important contribution to
the Nernst thermopower. We demonstrate that $\alpha_{xy}$ of a free
electron gas can be expressed purely and exactly as the entropy per
carrier irrespective of temperature (which agrees with seminal Hall
bar result of Girvin and Jonson). In two dimensions we prove the
universality of this result in the presence of disorder which
allows explicit demonstration of a number features of interest to
experiments on graphene and other two-dimensional materials. We
also exploit this relationship in the low field regime and to
analyze the rich singularity structure in $\alpha_{xy}(B, T)$ in
three dimensions; we discuss its possible experimental
implications. [Preview Abstract]

The concept of cooling a 2D electron gas (2DEG) using energy-selective
transport through quantum dots was first proposed by Edwards et al. [1].
Their scheme utilized two dots: one to remove hot electrons from an isolated
2DEG, the other to remove hot holes. The resulting current removes heat from
the 2DEG and dissipates it in adjacent reservoirs. We will present
measurements of a device designed to cool a 6$\mu $m$^{2}$ 2DEG using this
scheme [2]. The measurements reveal a complication not previously
considered: the charging energy of the cooled 2DEG itself. We will outline a
model that accounts for this, and shows that the device can still achieve
cooling. We will also show how the temperature of the cooled region can be
inferred from the line-shape of the current through the device. By comparing
measured line-shapes with predictions, we find the data to be consistent
with cooling of the isolated 2DEG by over 90mK in the best case.
[1] Edwards et al. Phys. Rev. B 52(8) p5714 (1995)
[2] Prance et al. Phys. Rev. Lett. 102, 146602 (2009) [Preview Abstract]

We present results on the non-equilibrium Fermi edge singularity
(FES)
problem in tunnel junctions. The FES, which is present in a Fermi
gas
subject to any sudden change of potential, manifests itself in
the final
state many body interaction between the electrons in the leads
[1]. We
establish a connection between the FES problem in a tunnel
junction and the
Full Counting Statistics (FCS) for the device [2]. We find that
the exact
profile of the changing potential (or the profile for the barrier
opening
and closing in the tunnel junction case) strongly affects the
overlap
between the initial and final state of the Fermi gas. We
factorize the
contribution to the FES into two approximately independent terms:
one is
connected with the short time opening process while the other is
concerned
with the long time asymptotic effect, namely the Anderson
orthogonality
catastrophe. We consider applications to a localized level
coupled through a
tunnel barrier to a 1D lead driven out of equilibrium [3].
References:
[1] G. Mahan, Phys. Rev. 163, 1612 (1967); P. Nozieres and C. T. De
Dominicis, Phys. Rev. 178, 1079 (1969); P. Anderson, Phys. Rev.
Lett. 18,
1049 (1967)
[2] J. Zhang, Y. Sherkunov, N. d'Ambrumenil, and B. Muzykantskii,
ArXiv:0909.3427
[3] D. Abanin and L. Levitov, Phys. Rev. Lett. 94, 186803 (2005) [Preview Abstract]

A low-temperature upturn of the Coulomb drag resistivity $\rho _{D
}$measured in undoped electron-hole bilayer devices, possibly
manifesting
from formation of a superfluid condensate or density modulated
state, was
recently observed. Here the effects of perpendicular and parallel
magnetic
fields on the drag upturn are examined. Measurements of $\rho
_{D}$ and
drive layer resistivity $\rho _{xx-e}$ as a function of
temperature and
magnetic field in two uEHBL devices are presented. In B$_{\bot
}$, the drag
upturn was enhanced as the field increased up to roughly .2 T,
beyond which
oscillations in $\rho _{D}$ and $\rho _{xx-e}$, reflecting Landau
level
formation, begin appearing. A small phase offset between those
oscillations,
which decreased at higher fields and temperatures, was also
observed. In
B$_{\vert \vert }$, the drag upturn magnitude diminished as the
field
increased. Above the upturn regime, both $\rho _{D}$ and $\rho
_{xx-e}$
were enhanced by B$_{\vert \vert }$, the latter via decreased
screening of
the uniform background impurities. This work has been supported
by the
Division of Materials Sciences and Engineering, Office of Basic
Energy
Sciences, U.S. Department of Energy. Sandia is a multiprogram
laboratory
operated by Sandia Corporation, a Lockheed Martin Company, for
the United
States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract]

Nuclear spin relaxation measurements are emerging as an effective
tool to study the low-frequency spin dynamics of two-dimensional
electrons in quantum Hall systems. We present such a measurement
in a two-dimensional electron system consisting of two filled
subbands. A recently developed pump-and-probe technique [1] was
used to obtain the nuclear spin relaxation time in a region where
the two sets of Landau levels, corresponding to the two subbands,
were nearly degenerate. The pump-and-probe technique allowed us
to measure the relaxation time over a much broader range of
magnetic fields and electron densities than that in an earlier
nuclear magnetic resonance study [2]. An array of interesting
observations will be reported,including phase space and
temperature dependence study. The project is supported by the NSF
under Grant No. DMR-0804794.
\\[4pt]
[1] N. Kumada, K. Muraki, and Y. Hirayama, Science 313, 329
(2006).
\\[0pt]
[2] X. C. Zhang, G. D. Scott and H. W. Jiang, Phys. Rev. Lett.
98,246802 (2007). [Preview Abstract]

We have used standard low frequency ac transport measurements,
as well as large bandwidth rf reflectometry measurements, to
study the non-equilibrium relaxation of the resistance of
strongly interacting, high
quality 2D GaAs hole systems at milliKelvin temperatures. We
observe logarithmic relaxation of the resistance over 7 orders
of magnitude in time (from 1ms to 10,000s) following a
discontinuous step in the gate voltage. This is characteristic
of glassy behaviour, and may be evidence for the formation of a
Coulomb glass. A comparison of the logarithmic behaviour
observed in different samples provides clues as to the origins
of these slow glassy dynamics. [Preview Abstract]

We demonstrate that three conductance features, 0.5 and 0.7G0
plateaus and a dip at 0.5G0, observed in quantum point contacts
(QPCs) can be consistently explained by the Rashba interaction
in the nonuniform electric field created by the side gates
along the transport direction. A quantity is defined which
depends on the extent of this nonuniformity and the electron
density. A short QPC tends to have a small . Only when is
large will the Rashba interaction produce a potential well deep
enough to localize the electron. This provides the bound state
that forms the Kondo resonance with the tunneling electrons. We
propose to compare the medium/long QPC to small/large quantum
dots, which are governed by the Kondo physics and the Coulomb
blockade, respectively. The relation between 0.7 anomaly and
the side-gate voltage, length of QPC, and temperature can all
be determined to agree qualitatively with the experiments. [Preview Abstract]

We report a giant room temperature piezoresistance in planar
metal/semiconductor hybrid resistors fabricated from Aluminum and
Silicon, with gauge factors $\sim $1000 for strains up to
10$^{-5}$. This new effect [1] is shown to be due to the
geometric arrangement of the metal and the semiconductor, and
results from a stress induced redirection of the injected current
from the metallic shunt into the semiconductor. Since there is a
large difference in the electrical conductivity of these
materials, this
yields a large increase in the device resistance. This
``extraordinary piezoconductance'' will be compared and
contrasted with the extraordinary magnetoresistance previously
observed in metal/semiconductor hybrid resistors.
\\[4pt]
[1] A.C.H. Rowe et al., Phys. Rev. Lett. 100, 145501 (2008)
[Preview Abstract]